The prior art marine propulsion devices such as outboard motors have commonly included an engine including a thermostat, the thermostat allowing the cooling system water to warm to a preset temperature when the engine is running at low speeds, and maintaining that temperature by opening and closing a water inlet port to thereby allow relatively small amounts of fresh cooler water to enter the engine, thereby maintaining the system at the desired temperature. At higher engine speeds it is desirable that the engine run at cooler temperatures. Accordingly, substantially increased amounts of cooler fresh water must be introduced into the engine. This has been accomplished in the prior art arrangements by the provision of a relief valve which is placed in parallel with the thermostat and which permits increased amounts of water to be forced into the engine at the high engine speeds. The water pump of the engine is driven by the engine, and the speed of the water pump and the water pressure produced by the water pump are, therefore, proportional to the engine speed. Once the engine reaches an increased speed, the water pressure produced by the water pump is sufficient to cause the spring loaded relief valve to open and to thereby cause an increased flow of water into the engine.
One of the features of the prior art systems is that two separate valves are required, the thermostat and the pressure relief valve. Additionally, the spring loaded or spring biased pressure relief valve restricts the flow of water through the engine since it is biased toward a closed position. Furthermore, the opening of the relief valve is controlled by the pump pressure and is not directly dependent upon the engine speed.
One example of an engine cooling system valve arrangement is illustrated in U.S. patent application Ser. No. 231,982, filed Feb. 6, 1981, that application being titled "Cooling System With Removable Valve Member" and being assigned to the assignee of the present invention.
Attention is also directed to the U.S. Nallinger Pat. No. 2,622,572, issued Dec. 23, 1952; the U.S. Woods Pat. No. 2,816,711, issued Dec. 17, 1957; the U.S. Middleton Pat. No. 2,833,478, issued May 6, 1958; the U.S. Woods Pat. No. 2,884,198, issued Apr. 28, 1959; French Pat. No. 1,137,376 and German Pat. No. 885,789.